“The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project ﬁnish date or violating a schedule constraint.”

So this is talking about flexibility we have at project level. Now in a network diagram, we can have multiple paths to reach the end. So this float talks about flexibility one path has as compared to others.Simple formula to calculate total float is our usual formula i.e. LS (Late Start) – ES (Early Start) or LF (Late, Finish) – EF (Early Finish)

Free Float

“The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint”

A method used to estimate the minimum project duration and determine the amount of scheduling flexibility on the logical network paths within the schedule model. This schedule network analysis technique calculates the early start, early finish, late start, and late finish dates for all activities without regard for any resource limitations by performing a forward and backward pass analysis through the schedule network. The critical path method is used to calculate the amount of scheduling flexibility on the logical network paths within the schedule model.